Electronic parts such as an IC memory element are sealed with a metal cap or a ceramic shell or sealed with a protective sealing material of organic resin, etc., by molding in order to increase it's reliability. Of such conventional methods, a technique of sealing electronic parts with an epoxy resin or other resin through molding or coating attracts attention from the standpoints of its good workability (production efficiency) and low cost, and is being put in practical use. However, when electronic parts sealed with an epoxy resin through molding or coating are subjected to heat cycle tests, since the epoxy resin generally has a high modulus of elasticity, the epoxy resin tends to give stress to the electronic parts so that the electronic parts often suffer damage or change in performance. For this reason, in sealing electronic parts with an epoxy resin or other resin through molding or coating, a method is employed in which a predetermined surface of an electronic part is first covered with a layer of a silicone rubber or silicone gel as a primary protective material, and this primary protective material layer is then covered with an epoxy resin or other resin through molding, etc. Particularly, since the silicone gel has a low modulus of elasticity and shows good adhesion to an epoxy resin or other resins due to its surface tackiness, silicone gel is often used as such a primary protective material for sealing minute electronic parts through molding.
As described above, the silicone gel has good tackiness and follow-up properties as compared with the silicone rubber and, hence, is suitable for use as a primary protective material. However, the silicone gel is so low in dielectric breakdown voltage that such is defective in insulating properties. To overcome such a problem, a method is employed in which a filler such as a silica powder is added to the above-described silicone gel. This method, however, is disadvantageous in that not only are the tackiness and follow-up properties inherently possessed by the silicone gel impaired, but also the pourability of the resulting silicone gel composition becomes poor to impair its workability, due to the sedimentation of the filler or viscosity increase before gelation or curing.
Another method is employed in which before a layer of the silicone gel is formed on an electronic part, a layer of a silicone rubber is formed beforehand as a primer layer. However, this method has a disadvantage in that the process necessarily includes a multi-stage molding operation and is complicated, although dielectric strength can be improved.